Selecting vertices algorythmically,i.e. a sphere?
import bpy,bmesh
from mathutils import Vector

mesh=bmesh.from_edit_mesh(bpy.context.object.data)
for v in mesh.verts:
    # select by sphere radius
    if v.co.length &lt; 8:
        v.select = True
    # select by cylinder radius
    vcyl = Vector((v.co.x,v.co.y,0))
    if vcyl.length &lt; 5:
        v.select = True
# trigger viewport update
bpy.context.scene.objects.active = bpy.context.scene.objects.active


See Blender: Scale a group of vertices using python on how to determine the center.

Demo: